Brokenness
The altar the speaker raises in his heart is described in the first line as “broken.” This is an expression of humility—like stones, the speaker’s heart is “hard” like “cement,” and in fragments. Because of this, he asks for God’s sanctification of his unworthy heart. Similarly, the stones that make up the altar have not been cut by any workman. In following Biblical precedent that forbids the use of tools on altar-stones, brokenness also implicitly means closeness to God's law. The only force that has the power to “cut” the stones is God.
Sacrifice
Sacrifice is an important motif in the poem. Traditionally, altars are used for ritual sacrifice. For a poet of Herbert’s religious beliefs, Christ’s crucifixion is the most important example of sacrifice. The last lines of the poem suggest that the speaker is trying to model himself on Christ's sacrifice. By asking for “thy blessed SACRIFICE to be mine” he asks for God’s permission to have his heart/his poem also accepted as a sacrifice.
Humility/pride
Throughout the poem the speaker attempts to humble himself before God, describing his heart in various self-deprecating ways. Despite the speaker's admission of human weakness and unworthiness, the whole poem is asking something of God: he wants God to accept his sacrifice. By describing this sacrifice of his heart as potentially similar to Christ’s sacrifice, he also shows an intense faith in his abilities to praise God eloquently. This prideful aspect of the poem is revealed by the fact that its shape not only resembles an altar, but also an upper-case “I.”
